1. Field of the Invention
Embodiments of the invention disclosed herein relate to methods and compositions for bypassing the involvement of CD4+ cells when generating antibody and MHC class I-restricted immune responses, controlling the nature and magnitude of the response, and promoting effective immunologic intervention in viral pathogenesis. More specifically, embodiments relate to immunogenic compositions for vaccination particularly therapeutic vaccination, against HIV and other microbial pathogens that impact functioning of the immune system, their nature, and the order, timing, and route of administration by which they are effectively used.
2. Description of the Related Art
The Major Histocompatibility Complex and T Cell Target Recognition
T lymphocytes (T cells) are antigen-specific immune cells that function in response to specific antigen signals. B lymphocytes and the antibodies they produce are also antigen-specific entities. However, unlike B lymphocytes, T cells do not respond to antigens in a free or soluble form. For a T cell to respond to an antigen, it requires the antigen to be bound to a presenting molecule known as a major histocompatibility complex (MHC) antigen/protein/marker.
MHC proteins provide the means by which T cells distinguish healthy “self” cells from foreign or infected (non-self) cells. MHC molecules are a category of immune receptors that present potential peptide epitopes to be monitored subsequently by the T cells. There are two types of MHC, class I MHC and class II MHC. CD4+ T cells interact with class II MHC proteins and predominately have a helper phenotype while CD8+ T cells interact with class I MHC proteins and predominately have a cytolytic phenotype, but each of them can also exhibit regulatory, particularly suppressive, function. Both classes of MHC protein are transmembrane proteins with a majority of their structure on the external surface of the cell. Additionally, both classes of MHC have a peptide binding cleft on their external portions. It is in this cleft that small fragments of proteins, native or foreign, are bound and presented to the extracellular environment.
The antigen receptor of T cells, or T cell receptor (TCR), recognizes the complex formed by peptide and MHC marker by binding to it. The MHC is highly polymorphic with the result that the specificity exhibited by a TCR is dependent on both the peptide and the MHC marker in the recognized complex. This requirement is called MHC restriction. T cell immune responses are induced when T cells recognize peptide-MHC marker complexes displayed by cells called professional antigen presenting cells (pAPCs). Effector functions, such as cytolytic activity or cytokine secretion, are actuated when T cells subsequently recognize peptide-MHC marker complexes on other cells of the body.
HIV
Human Immunodeficiency Virus (HIV) is a member of the Lentivirus genus of the Retroviridae family. This family of viruses is known for latency, persistent viremia, infection of the nervous system, and weak host immune responses. HIV has high affinity for CD4+ T lymphocytes and monocytes. HIV binds to CD4+ T-cells at the cell surface and becomes internalized. The virus replicates by generating a DNA copy by reverse transcriptase. Viral DNA becomes incorporated into the host DNA, enabling further replication. HIV is the causative agent of acquired immune deficiency syndrome, AIDS.
Despite more than 20 years of HIV related research, infection with HIV remains a major public health concern. Globally, more than 42 million people are infected, including about 5 million newly infected in the year 2003 (Garber, D. et al., The Lancet Infectious Diseases 4:397-413, 2004). The most common clinical manifestations of HIV are due to progressive immunodeficiency caused by a selective loss of CD4+ lymphocytes (Buckland, M. S. & Pinching, A. J. Intern. J of STD & AISA 15:574-583, 2004). Both CD4+ and CD8+ T-cells are important in the control of viral, including HIV, replication. Activated CD4+ T-helper cells produce cytokines and interact with cell-surface receptors that prompt B cells to produce antibodies, and they interact indirectly (via antigen presenting cells) or directly with CD8+ T lymphocytes to induce differentiation into cytotoxic cells. HIV grows far better in activated cells than in cells at rest (Roberts, J. P. The Scientist 18:26-27, 2004). Consequently, the very cells central in orchestrating the fight against viral pathogens, CD4+ T-cells, may then be lost by apoptosis, cytolysis, or cell mediated cytotoxicity. The result is an ineffective immune response due to the prompt deletion of activated T cells, with expected repercussions on induction, expansion and differentiation of CD8+ T cells and B cells recognizing viral antigens. Initially, the rate of production of CD4+ T cells is greater than peripheral destruction, and so antibody production and generation of an expanded repertoire of CD8+ T-cells to kill virally infected targets proceeds correctly. Over time, the rapid mutation rate of HIV, poor immunogenic characteristics of HIV proteins, and the scale of HIV replication overwhelm the host immune system. Since CD4+ T cells are required to support the pool of HIV-specific CD8+ T-cells, the loss of HIV-specific CD4+ cells leads to a loss of HIV-specific CD8+ T-cells. Immune containment of HIV infection fails and clinical progression to AIDS ensues.
Most infected patients do not exhibit overt clinical manifestations of the disease for six to ten years following initial infection. However a small group remain long-term non-progressors (LTNP), and remain free of disease for ten or more years. They exhibit lower viral loads and stable CD4+ cell counts which have in part been attributed to cell-mediated immunity. The nature of viral suppression in this group has been the focus of much research. There has been a great deal of effort made to understand the characteristics of LTNP and the mechanism by which the disease-free state is achieved, so that better therapeutics and prophylactics may be designed.
Therapeutics
Morbidity and mortality associated with HIV infection have been dramatically reduced with the advent of antiretroviral therapy targeting two key enzymes: reverse transcriptase and protease. However, beneficial effects can be variable, prolonged treatment induces considerable toxicity, and effectiveness is undermined by the emergence of drug-resistant mutations. Also, the high cost of antiretroviral therapies limits access and availability in developing countries. Thus, alternative, less costly strategies capable of effecting sustained viral suppression are desperately needed.
Therapeutic immunization as a treatment for HIV infection may prove to be such an alternative. However, several critical aspects of HIV infection present novel challenges to the development of an effective vaccine. These properties include viral particles that are difficult to neutralize with antibodies; selective infection, destruction, and impaired regeneration of CD4+ T-helper cells; rapid virus evolution providing escape from cellular and humoral immune responses; and high viral genetic diversity, distribution, and prevalence (Garber, D. et al., The Lancet Infectious Diseases 4:397-413, 2004). Also, it has been recently suggested that a vaccine-primed immune system might be more susceptible to infection. Boosting the HIV specific helper cells, an outcome of vaccination, may be giving the virus more targets to infect. Since more conventional vaccination strategies depend on co-induction of T helper (Th) cells, it is expected that their efficacy is low or the overall effect detrimental in a setting where Th cell function is impaired by HIV (Roberts, J. P. The Scientist 18:26-27, 2004).
The immune system may effectively eliminate virus-infected cells during the clinical course of HIV-1 infection using virus-specific major histocompatibility complex (MHC) class-I restricted CTL activity (Koup, et al. J Exp Med. 180 (3):779-82, 1994; Koup et al. “Nature, 370 (6489):416, 1994; and Koup et al., J. Virol. 68 (7):4650-5, 1994). There is evidence that suggests HIV-1-specific CTL activity is important for controlling viral spread during the clinical course of HIV-1 infection (Klein, 1995; Koup, 1994), for maintaining low levels of viral load during the asymptomatic phase (Musey, 1997; Rinaldo, 1995; Koup, 1994; Walker, 1987), and possibly for complete elimination of virus-infected cells, as implied from the observation of HIV-exposed, but virus-negative, children and women (Rowland-Jones, 1995; Rowland-Jones, 1993). Furthermore, observations from cross-sectional studies have shown the absence, or severely decreased levels, of HIV-1-specific CTL responses during advanced stages of HIV-1 infection (Carmichael, 1993). Taken together, recent vaccine strategy has focused on eliciting antiviral CD8+ T cell responses to control the level of HIV replication in vivo (Garber, D. et al., The Lancet Infectious Diseases 4:397-413, 2004). Rationale for potential efficacy of CD8+ T-cell-based AIDS vaccine is that reduction of the level of setpoint viral load may slow the rate of progression to AIDS and eliminate active reservoirs of infection.
Other Pathogens
HIV is not the only pathogen for which activation or expansion of CD4+ cells is associated with pathological processes. For example corneal scarring incident to herpes simplex virus (HSV) infection is attributable to the action of CD4+ T cells and the cytokines they produce. (Osorio, Y. et al., Ocul. Immunol. Inflamm. 10: 105-116, 2002; Altmann, D. M. & Blyth, W. A. J. Gen. Virol. 66:1297-1303, 1985; Xu, M. et al., J. Immunol. 173:1232-1239, 2004). HIV is also not the only pathogen for which impairment of the CD4+ T cell response results in failure to mount a more effective immune response, persistence of infection, and greater morbidity or mortality. Failure of dendritic cells (DC) to increase class II MHC expression, and thus productively interact with CD4+ T cells, contributes to the persistence of Hepatitis B virus (HBV) infection (Zheng, B. J., et al., J. Viral Hepat. 11:217-224, 2004; Lohr, H. F., et al., Clin. Exp. Immunol. 130:107-104, 2002). Similarly impaired DC-CD4+ T cell interactions are involved in the poor immune responses to and persistence of infection by Hepatitis C virus (HCV) (Murakami, H., et al. Clin. Exp. Immunol. 137:559-565, 2004).
Embodiments described herein relate to methods and compositions that alleviate or overcome the above-described challenges associated with the treatment of microbial infections, including those associated with HIV, herpes simplex virus (HSV), HBV, HCV, hepatitis G virus (HGV), human papilloma virus (HPV), cytomegalovirus (CMV), influenza virus, human T-cell leukemia virus (HTLV), Respiratory syncytial virus (RSV), Epstein Barr virus (EBV), measles virus, and Ebola virus, for example.